A furnace running too often is a problem. This is a condition known as Short Cycling. In order to avoid it, we need to actually know how long a system should be cycling. How do we know it’s been cycling too often? How often should a furnace actually run? Does it matter?
The Big Variables
The technical answer to this is always: “consult the manufacturer specifications.” The real answer is buried in a mess of factors we’ll need to weed out. The first big challenge in this problem is the variability of our environment. During the summer months, a furnace could be shutdown all day and that would be just fine. For those times, it’s just not needed. On the flip side of the coin however, during the peak of winter, the furnace needs to be able to keep up with the heating needs of the building without running itself into complete failure.
In order to keep up with the demand for heat, a furnace needs to be the right size. Even then, we need to allow for times of extreme demand such as on the coldest day of the year. We can’t quite get away with saying “if it ever runs all day, that’s a problem.” We also need to account for just how hot the thermostat is set too. If someone from perhaps Mexico were to venture into the freezing environment of Toronto, Canada, they might well set the thermostat to a relatively boiling 90 degrees.
We’re left with this juggling act of efficiency, margin in performance, actual user demand, and system design. An ideal system might be designed to run up to five times in an hour, at the user’s peak demand. A smaller system might never reach the target temperature. An oversized system might send a pulse of heat, trip the thermostat, and be tricked into short cycling. Any system constantly switching states or just running in desperation to catch up is going to be inefficient.
No Perfect Number
In a perfect world, you want your system to run within the manufacturer’s specifications for ideal efficiency. This is probably 3-5 cycles per hour and no more. Due to mother natures’ varying wrath and users’ varying whims, no system will ever be perfectly efficient year-round.
If your furnace cycles six times an hour, on the colder days of winter, but maybe it has to run longer or cycle more frequently like eight times in an hour that’s alright as long as it reaches a comfortable temperature inside and everyone’s happy. On the flipside, if the furnace is running constantly through the winter. If it has to cycle ten times in an hour, it’s time to call that what it is: short cycling. It’s time to consult the manual and contact your HVAC Contractor, something’s not right.
Handling the Problem
Your furnace is running too often, but everything checks out. No bad safeties and it’s not trying to self-destruct. Now what?
Let’s look at those variables again. If the system can never keep up, the weather is average, and you know demand for heat won’t go down, it’s time to buy a bigger system or augment the existing hardware. On the flipside, if the hardware is too big for the job, it may be time to consult a contractor about modifying the furnace programming, modifying the thermostat, or finding some way to compensate for the over performing hardware.
If there is a particularly nasty cold front rolling through, and this sort of event is rare, you have a few options. You can lower the thermostat to lessen the demand on the furnace for a while. You can augment the furnace with portable electric heaters. At worst, you can live with the additional fuel costs and wear on the hardware for perhaps a few days and weeks until mother nature eases up.
Before making these decisions, remember that furnace run time is not just a number in a manual. It’s a plethora of factors about the facility, the occupants, and the local climate.